This invention relates to high power linear amplifiers and in particular relates to the same using digital pre-distortion.
First and second generation cellular systems have historically used forms of modulation which are either constant envelope (e.g. GMSK in GSM) or which result in relatively low levels of amplitude modulation. The linearity of the high power amplifiers used for such systems has therefore not been an important technical issue; indeed, for the constant envelope systems it is standard practice to operate the amplifiers either close to or actually in compression in order to maximise power efficiency.
Third generation cellular systems however typically use linear spread-spectrum modulation schemes with a large amount of amplitude modulation of the signal envelope. When passed through a high power amplifier, the output is typically distorted in amplitude and phase by the non-linearity of the amplifier: the amplitude and phase distortion effects are commonly referred to as AM-AM conversion and AM-PM conversion respectively. Both distortion effects are a function only of the amplitude envelope of the input signal and are insensitive to the input phase envelope.
In systems such as Code Division Multiple Access (CDMA) modulation schemes, a plurality of signals are transmitted in a communication system and are amplified simultaneously. When a plurality of signals are applied to a linear amplifier, its non-linear characteristics will tend to produce interaction between the signals being amplified and the amplifier output will contain intermodulation products. Such intermodulation products reduce signal quality by allowing cross-talk to occur and such spillage often falls outside a particular licensed spectrum and must be controlled. Such intermodulation distortion can be reduced by negative feedback of the distortion components, pre-distortion of the signal to be amplified to cancel the amplifier generated distortion, or by separating the distortion components with the amplifier output and feeding forward the distortion component to cancel the distortion of the amplifier output signal.
There are many ways of linearising a high power amplifier: direct RF feedback, envelope feedback, feed-forward and pre-distortion. For cellular power amplifiers, feed-forward amplifiers are commonly used. Feed forward amplifiers are more complicated in that they require the modification of the separated distortion component in amplitude and phase to match the gain and phase shift of the amplifier on a continuous basis and require an error amplifier which is typically similar in power handling to the main amplifier which incurs a heavy penalty in RF device cost and power efficiency.
Envelope feedback methods (polar and Cartesian) perform much better than feed-forward amplifiers in terms of device cost and efficiency since the RF signal linearistaion processing is done before the power amplifier on a small signal. However, envelope feedback is fundamentally limited in the correction bandwidth obtainable by the delay of the feedback loop. As systems migrate to wider band modulation (e.g. CDMA2000 and WCDMA) a linearisation technology is required which is fundamentally a wideband technique.
Most implementations of pre-distortion are inherently wideband, however the performance achievable has been limited by the difficulty of matching the complex distortion characteristics of typical power amplifier devices with simple analogue pre-distortion networks.
U.S. Pat. No. 4,700,151 (Nagata) provides a baseband (analogue or digital ) modulation system and technique which employs a look-up table for adaptation. U.S. Pat. No. 5,049,832 (Cavers) provides a digital pre-distortion arrangement which reduces memory requirements to under 100 complex pairs, with a resultant reduction in convergence time and removes the need for a phase shifter or PLL in a feedback path.
The applicant""s U.S. patent application Ser. No. 09/209,386 filed on Dec. 10, 1998 provides a method of linearising a power amplifier by pre-distorting the input to correct for the instantaneous non-linear transfer function of a power amplifier. The input is split into two paths. The main path goes through a delay line into a modulator and then into an amplifier. The second signal is sent to an amplitude detector whose output is converted into a digital number and used to address a look-up table. The look-up table output is converted into an analogue signal which is used to modulate the main signal such that the power amplifier is linearised. Although this does reduce unwanted emissions out of the licensed bandwidth, further reductions without incurring much extra cost are desirable.
The present invention seeks to provide an improved linear amplifier arrangement which achieves correction over a wide bandwidth with low system cost and high efficiency. More particularly the present inventIon seeks to provide a linear amplifier arrangement capable of amplifying and combining a number of frequency carriers or bearers.
According to a first aspect of the present invention there is provided an amplifier arrangement comprising:
an amplifier for amplifying an input radio frequency (RF) signal to generate an output RF signal;
a linear function processor for implementing a linear function of previous samples of the input signal to generate a linear function correction signal; and
a pre-distortion processor for distorting the current input signal in accordance with the linear function correction signal in order to compensate for distortion in the output signal.
As well has exhibiting instantaneous amplitude and/or phase distortion effects a power amplifier will generally exhibit distortion effects based on the past history of the input signal. For example amplitude and/or phase distortion in the amplifier may be different when the input sample has just undergone a rapid reduction in amplitude than when the input signal has just undergone a rapid increase in amplitude. These historical distortion effects can be greatly reduced while adding little extra cost to the amplifier arrangement by applying a pre-distortion to the input signal based on a linear function of past samples of the input signal.
It should be noted that the pre-distortion processor may take into account other inputs than the linear function when distorting the input signal.
The linear function processor may by a finite impulse response filter and the linear function may have the form:       g    ⁡          (      t      )        =            ∑              K        =        0            N        ⁢          xe2x80x83        ⁢                  h        ⁡                  (          k          )                    ⁢              "LeftBracketingBar"                  x          ⁡                      (                          t              -              k                        )                          "RightBracketingBar"            
where
g(t) is the linear function,
h(k) is a scaling factor,
|x(txe2x88x92k)| are the previous samples of the input signal, and
N is the number of previous samples
The fact that g(t) is a linear function of the past samples of the input signal |x(txe2x88x92k)| makes the pre-distortion easy to implement in a cost effective manner. A particularly simple and yet effective implementation is provided when the linear function has the form:
h(t)=K(|x(txe2x88x922)|xe2x88x92|x(txe2x88x923)|)
where
h(t) is the linear function,
K is a scaling factor,
|x(txe2x88x922)| is the previous sample of the input signal two samples before the current input signal, and
|x(txe2x88x923)| is the previous sample of the input signal three samples before the current input signal.
A linear function processor for implementing this linear function can be formed using three delay lines, a subtractor and a multiplier. This can enable the linear function processor to be implemented in spare space in existing processing hardward already used in other parts of the amplifier arrangement.
Where the memory dependent distortion effects in the amplifier predominantly effect the amplitude of the output signal the linear function processor may be an amplitude linear function processor for generating an amplitude linear function correction signal to compensate for amplitude distortion in the amplifier. Then the pre-distortion processor generates an amplitude correction signal in accordance with the amplitude linear function correction signal and amplitude modulates the input signal with the amplitude correction signal.
Where the memory dependent distortion effects in the amplifier predominantly effect the phase of the output signal, the linear function processor may be a phase linear function processor for generating a phase linear function correction signal to compensate for phase distortion in the amplifier. Then the pre-distortion processor generates a phase correction signal in accordance with the phase linear function correction signal and phase modulates the input signal with the phase correction signal.
Where the memory dependent distortion effects in the amplifier effect the both that amplitude and the phase of the output signal, the linear function processor may comprise an amplitude linear function processor for generating an amplitude linear function correction signal to compensate for amplitude distortion in the amplifier and a phase linear function processor for generating a phase linear function correction signal to compensate for phase distortion in the amplifier. Then the pre-distortion processor generates an amplitude correction signal in accordance with the amplitude linear function correction signal, generates a phase correction signal in accordance with the phase linear function correction signal, amplitude modulates the input signal with the amplitude correction signal and phase modulates the input signal with the phase correction signals.
In a preferred implemuentation the linear function processor comprises:
a first delay line for delaying the input signal by one cycle;
a subtractor for subtracting the delayed input signal from an undelayed input signal;
second and third delay lines for each delaying the subtracted signal by one cycle; and
a multiplier for multiplying the subtracted signal by a scaling factor.
This type of arrangement can enable the linear function processor to be implemented in spare space in existing processing hardward already used for other functions of the amplifier arrangement.
In a more complex, but also more flexible implementation the linear function processor may comprise:
a plurality of delay lines arranged in a series for delaying the input signal successively by one cycle;
a plurality of multipliers for multiplying by a scaling factor a respective one of a plurality of tapped signals, selected from the input signal and the outputs of any one of the delay lines; and
an adder for adding the outputs from the multipliers.
The linear function processor is preferably a digital processor. A part of the input analogue signal to the amplifier is tapped off the main input signal is detected by an envelope detector, filtered and then converted to digital form by an analogue to digital converter. The resulting digital signal is input into the linear function processor. The output from the linear function processor is further processed by the pre-distortion processor and converted into analogue form and then used to modulate the main input signal.
According to a second aspect of the present invention there is provided an amplifier arrangement comprising:
an amplifier for amplifying an input radio frequency (RF) signal to generate an output RF signal;
a linear function processor for implementing a linear function of previous samples of the input signal to generate a linear function correction signal;
a non-linear function processor for implementing a non-linear function of the input signal to generate a non-linear function correction signal; and
a pre-distortion processor for distorting the current input signal in accordance with the linear and non-linear function correction signals in order to compensate for distortion in the output signal.
According to the second aspect of the invention a non-linear function processor is provided for compensating for instantaneous distortion effects in the amplifier. The non-linear function processor may be implemented as a look-up table. In a preferred version the pre-distortion processor distorts the input signal in accordance with a sum of the linear function correction signal and the non-linear function correction signal.
The non-linear function processor preferably provides compensation for amplitude and phase distortion in the output signal and so generates an amplitude non-linear function correction signal for compensating for amplitude distortion and an phase non-linear function correction signal for compensating for phase distortion.
The second aspect of the present invention the same preferred features recited above in relation to the first aspect of the present invention.
According to a third aspect of the present invention there is provided a method of linearising an amplifier for amplifying an input radio frequency (RF) signal to generate an output RF signal, comprising the steps of:
generating a linear function correction signal which is a linear function of previous samples of the input signal; and
distorting the current input signal in accordance with the linear function correction signal in order to compensate for distortion in the output signal.
The third aspect of the present invention has the same advantages discussed above in relation to the first aspect of the present invention.
According to a fourth aspect of the present invention there is provided a method of linearising an amplifier for amplifying an input radio frequency (RF) signal to generate an output RF signal, comprising the steps of;
generating a linear function correction signal which is a linear function of previous samples of the input signal;
generating a non-linear function correction signal which is a non-linear function of the input signal; and
distorting the current input signal in accordance with the linear and non-linear correction signals in order to compensate for distortion in the output signal.
According to a fifth aspect of the present invention there is provided an amplifier arrangement comprising:
an amplifier for amplifying an input radio frequency (RF) signal to generate an output RF signal;
an error detection processor responsive to the input signal and the output signal for generating an error correction signal which represents a difference between the input and output signals due to distortion in the amplifier,
an adaptive linear function processor responsive to the error correction signal for implementing a linear function of previous samples of the input signal to generate a linear function correction signal; and
a pre-distortion processor for distorting the current input signal in accordance with the linear function correction signal in order to compensate for distortion in the output signal.
Preferably, the linear function has the form:       g    ⁡          (      t      )        =            ∑              K        =        0            N        ⁢          xe2x80x83        ⁢                  h        ⁡                  (          k          )                    ⁢              "LeftBracketingBar"                  x          ⁡                      (                          t              -              k                        )                          "RightBracketingBar"            
where
g(t) is the linear function,
h(k) is a scaling factor,
|x(txe2x88x92k)| are the previous samples of the input signal, and
N is the number of previous samples,
and additionally comprises a digital signal processor for generating the scaling factors in response to the error correction signal from the error detection processor in accordance with a means squared error form of adaptation.
According to a sixth aspect of the present invention there is provided a method of linearising an amplifier arrangement for amplifying an input radio frequency (RF) signal to generate an output RF signal comprising the steps of;
generating an error correction signal which represents a difference between the input and output signals due to distortion in the amplifier in response to the input signal and the output signal,
adaptively implementing in response to the error correction signal a linear function of previous samples of the input signal to generate a linear function correction signal; and
pre-distorting the current input signal in accordance with the linear function correction signal in order to compensate for distortion in the output signal.